Kinslaying is the act of slaying a family member and a great taboo in the Seven Kingdoms. Whoever commits it is dubbed a kinslayer. Any individual who slays a member of their own family is believed to be cursed forever in the sight of gods and men.

The taboo associated with kinslaying is strong enough to stay the hand of even the most ruthless men. Tywin Lannister would have liked nothing more than to kill his son Tyrion the day he was born, because his mother died giving birth to him and for the shame he brought on their family for being born a dwarf. However, Tywin felt that House Lannister was above the shame and curse of kinslaying, so he relented and let Tyrion live.[1]

Melisandre later insists to Stannis that she must sacrifice King Robert's bastard son Gendry. Stannis is uneasy about this and Davos Seaworth points out why: Gendry is his own nephew and he would be spilling his own blood. Davos concedes that Renly had wronged Stannis, declared himself king ahead of his older brother, stole his rightful bannermen to raise an army, and Renly himself would have killed his brother Stannis when their forces clashed on the battlefield. Renly was no innocent, but Gendry is just an innocent bastard boy. Davos verbalizes Stannis' doubts, who hesitates whether to spare or kill Gendry as it would be an unlawful kinslaying.[5] Eventually the matter is taken out of Stannis' hands, when Davos helps Gendry escape.

King Robb Stark, is accused of it by Lord Rickard Karstark when the former is about to execute the latter. This was due to the distant blood ties between House Stark and House Karstark, and it is debatable if Robb would be officially labelled a kinslayer as a result.[7]

Lord Walder Frey committed kinslaying when he masterminded the massacre of House Stark at the Red Wedding, though for relatives by marriage. Walder's own daughter Roslin Frey married Robb's uncle Edmure Tully mere hours before the massacre began, making Edmure Walder's son-in-law, and Robb Walder's great-nephew-in-law.

In the books

In the A Song of Ice and Fire novels, the taboo against kinslaying is pretty much the only reason that neither Tywin nor Tyrion ever try to kill Joffrey. Soon after Joffrey is crowned king it rapidly becomes apparent that he isn't simply vicious and cruel: he's an utter imbecile. Moreover, Joffrey is so utterly impulsive that he publicly, brazenly commits acts of petty cruelty that even the most ruthless kings of the past would have hesitated to commit, simply because it would be counter-productive. After Joffrey needlessly starts a city-wide riot, Bronn openly asks Tyrion why they don't simply assassinate Joffrey given that he causes far more problems than he solves, and moreover, actually has a pliable younger brother (Tommen) whom the Lannisters could easily use to replace him as their puppet king. Essentially the only reason Tyrion gives is that he won't kill his own nephew.

Characters in Westeros who have never met Daenerys Targaryen, as they start to hear news of her string of conquests in Slaver's Bay, are sometimes appalled to hear that her husband Drogo killed her own brother Viserys as she watched. This is largely because they didn't know how much of a monster he was, or that he violated Dothraki law by drawing a sword in Vaes Dothraki and threatening Drogo's unborn son, and thus Drogo was entirely within his rights to execute him. Then again, in these discussions other characters point out the well-known madness that runs in the Targaryen family, and accurately guess that Viserys may have been not much better than his crazed father King Aerys II Targaryen.

In the books Stannis Baratheon never admits that he ordered Melisandre to send the shadow demon against Renly, thus it is unclear if he is actually guilty of kinslaying.

Theon Greyjoy is sometimes inaccurately accused of being a kinslayer for his alleged execution of Bran and Rickon Stark, though as a highborn hostage and ward of the Starks was not actually their blood relative.

"Alton Lannister" in the TV series is a renamed version of Cleos Frey from the books. Cleos's mother Genna is actually Tywin's sister, making Cleos a first cousin to Jaime. The TV series apparently renamed Cleos for fear that the audience would be confused why a man named "Frey" was fighting for the Lannisters in Season 2 (before the Freys turn on Robb Stark in Season 3), so they renamed him as a Lannister to make clear what side he is on. Jaime didn't kill Cleos Frey in the books, rather he accompanied Brienne of Tarth on the mission to return Jaime to King's Landing for a prisoner exchange, but he was killed by outlaw bandits along the way. The TV series later made clear in "A Man Without Honor", however, that "Alton Lannister" isn't exactly the same character as Cleos Frey, but is instead a far more distant cousin. Jaime has difficulty even remembering who Cleos's mother is, and Alton says she is Cynda Lannister, not Jaime's well-known aunt Genna. So on the one hand, Jaime was never a kinslayer in the books but the TV series has him kill Alton. On the other hand, the TV series changed the relationship of "Alton Lannister" so he actually isn't a very close relative of Jaime at all - it is debatable how much the shame of kinslaying would apply to a fifth or sixth cousin. Moreover, it's vaguely implied that "Alton" was willing to give his life in a heroic escape attempt if it would free the famous Jaime (though he didn't understand that when Jaime said Alton wouldn't survive the escape attempt, he meant that he would kill Alton to distract the guard).

Killing a relative by marriage, instead of a blood relative, is still considered kinslaying, because in a spiritual sense they have become part of the same family. It's possible that it isn't considered quite as bad but only by a matter of degree, i.e. killing your own brother or son is generally seen as somewhat worse than killing your own brother-in-law, though both are still reviled. Lord Tytos Blackwood's son was killed at the Red Wedding, and he notes that one of Lord Walder Frey's wives was herself a Blackwood, but apparently the bonds of marriage mean nothing to Lord Walder. This wife was actually the mother of Lame Lothar Frey, meaning that Lame Lothar was responsible for the death of his kinsman Lucas Blackwood - given Lothar's club foot he might not have participated in the fighting directly, but he was one of the main masterminds who orchestrated the massacre and gave the orders. The exact relationship between Lame Lothar's Blackwood mother and the main Blackwood family hasn't been given, though the TV series might have condensed this. Just as kinslaying is considered a worse crime than regicide, violation of guest right is considered to be an even worse crime than kinslaying, so any kinslaying which occurred at the Red Wedding is overshadowed in the narrative by how outrageous and unthinkable it was that Lord Walder murdered guests under his own roof.